Method of manufacturing nitride substrate, and nitride substrate

ABSTRACT

A method of manufacturing a nitride substrate includes the following steps. Firstly, a nitride crystal is grown. Then, the nitride substrate including a front surface is cut from the nitride crystal. In the step of cutting, the nitride substrate is cut such that an off angle formed between an axis orthogonal to the front surface and an m-axis or an a-axis is greater than zero. When the nitride crystal is grown in a c-axis direction, in the step of cutting, the nitride substrate is cut from the nitride crystal along a flat plane which passes through a front surface and a rear surface of the nitride crystal and does not pass through a line segment connecting a center of a radius of curvature of the front surface with a center of a radius of curvature of the rear surface of the nitride crystal.

TECHNICAL FIELD

The present invention relates to a method of manufacturing a nitridesubstrate, and a nitride substrate.

BACKGROUND ART

An MN (aluminum nitride) crystal has a wide energy band gap of 6.2 eV, ahigh thermal conductivity of about 3.3 WK⁻¹ cm⁻¹, and a high electricalresistance. Thus, nitride crystals such as an MN crystal have beenattracting attention as materials for semiconductor devices such asoptical devices and electronic devices.

A method of manufacturing such a nitride crystal is disclosed, forexample, in Japanese Patent Laying-Open No. 2007-197276 (Patent Document1). In Patent Document 1, a group III-V nitride semiconductor substrateis manufactured by the following steps. Specifically, a group III-Vnitride semiconductor film is grown on a substrate of a different typehaving a c-plane or an off angle. Thereafter, a metal film is depositedon the substrate of a different type, and heat treatment is performed toform cavities in the group III-V nitride semiconductor film. Next, agroup III-V nitride semiconductor crystal is deposited on the metalfilm. Subsequently, the substrate of a different type is delaminated toobtain a group III-V compound semiconductor crystal having a c-axissubstantially vertical to a front surface or inclined by a predeterminedangle with respect to the front surface. Next, a rear surface of thegroup III-V nitride semiconductor crystal is polished to obtain a flatsurface. A group III-V nitride semiconductor substrate is manufacturedby removing the substrate of a different type, the group III-V nitridesemiconductor film, and the metal film from the group III-V nitridesemiconductor crystal. A semiconductor substrate made of the group III-Vnitride semiconductor crystal manufactured as described above has anas-grown front surface.

PRIOR ART DOCUMENTS Patent Documents

Patent Document 1: Japanese Patent Laying-Open No. 2007-197276

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

In Patent Document 1, however, the group III-V nitride semiconductorcrystal is grown in an as-grown state. That is, an off angle of thefront surface of the group III-V nitride semiconductor substrate iscontrolled only by growth conditions of the group III-V nitridesemiconductor crystal. Thus, it has been difficult to manufacture agroup III-V nitride semiconductor substrate having a controlled offangle with a high yield.

Therefore, one object of the present invention is to provide a method ofmanufacturing a nitride substrate by which a nitride substrate having acontrolled off angle of a front surface is manufactured with an improvedyield, and the nitride substrate.

Means for Solving the Problems

A method of manufacturing a nitride substrate in one aspect of thepresent invention includes the following steps. Firstly, a nitridecrystal is grown. Then, the nitride substrate including a front surfaceis cut from the nitride crystal. In the step of cutting, the nitridesubstrate is cut such that an off angle formed between an axisorthogonal to the front surface and an m-axis or an a-axis is greaterthan zero.

The nitride substrate of the present invention is characterized in that,in the nitride substrate including the front surface, the off angleformed between the axis orthogonal to the front surface and the a-axisor the m-axis is greater than zero.

According to the method of manufacturing a nitride substrate and thenitride substrate in one aspect of the present invention, the nitridesubstrate is cut from the nitride crystal to have an off angle in theentire front surface. Thus, the nitride substrate having a controlledoff angle can be manufactured stably irrespective of the state of thegrown nitride crystal (that is, without depending on growth conditionsand the like of the nitride crystal). When an epitaxial layer is formedon the front surface of the nitride substrate manufactured as describedabove, step growth as crystal growth in a lateral direction can beperformed. Hence, the epitaxial layer can have good morphology, and thuscrystallinity can be improved. Since the crystallinity of the epitaxiallayer is improved, properties of a substrate, a device, and the likefabricated using the epitaxial layer can be improved. Therefore, anitride substrate including a front surface in which an off angle iscontrolled to form an epitaxial layer with high properties all over thenitride substrate can be manufactured with an improved yield.

A method of manufacturing a nitride substrate in another aspect of thepresent invention includes the following steps. Firstly, a nitridecrystal including a front surface and a rear surface opposite to thefront surface is grown in a c-axis direction. Then, the nitridesubstrate is cut from the nitride crystal. In the step of cutting, thenitride substrate is cut from the nitride crystal along a flat planewhich passes through the front surface and the rear surface of thenitride crystal and does not pass through a line segment connecting acenter of a radius of curvature of the front surface with a center of aradius of curvature of the rear surface of the nitride crystal.

As a result of earnest study, the inventors of the present inventionhave found that, when the nitride substrate is cut from the nitridecrystal along a flat plane which passes through the line segmentconnecting the center of the radius of curvature of the front surfacewith the center of the radius of curvature of the rear surface of thenitride crystal, a front surface of the nitride substrate includes aportion having an off angle of zero. Therefore, according to the methodof manufacturing a nitride substrate in another aspect of the presentinvention, a nitride substrate in which an off angle formed between anaxis orthogonal to a front surface and an a-axis or an m-axis is alwayscontrolled to be greater than zero can be manufactured. Further, sincethe nitride substrate is cut from the nitride crystal with the off anglebeing controlled, a nitride substrate having a controlled off angle canbe manufactured stably irrespective of the state of the grown nitridecrystal. Therefore, a nitride substrate in which an off angle iscontrolled to form an epitaxial layer with high properties can bemanufactured with an improved yield.

Preferably, in the method of manufacturing a nitride substrate describedabove, in the step of cutting, the nitride substrate is cut such thatthe nitride substrate includes the front surface having a first regionand a second region surrounding the first region, and the off angle hasa minimum value at a first point in the second region.

Further, preferably, in the nitride substrate described above, the frontsurface has a first region and a second region surrounding the firstregion, and the off angle of the front surface has a minimum value at afirst point in the second region.

Thereby, the off angle in the first region can be further increased, andthus step growth can be further enhanced when an epitaxial layer isformed on the nitride substrate. Hence, an epitaxial layer with higherproperties can be formed on the first region located on an innerperipheral side. In a substrate, a device, and the like fabricated usingan epitaxial layer, an outer peripheral side is generally lessfrequently used, and an inner peripheral side is generally morefrequently used. Therefore, since properties in a more frequently usedregion in the substrate, device, and the like can be further improved, anitride substrate in which an off angle is controlled to form anepitaxial layer with higher properties can be manufactured with animproved yield.

Preferably, in the method of manufacturing a nitride substrate describedabove, in the step of cutting, the nitride substrate is cut such thatthe second region is within 2 mm from an edge of the nitride substrate.Further, preferably, in the nitride substrate described above, thesecond region is within 2 mm from an edge.

Thereby, a wide region other than the second region within 2 mm from theedge of the nitride substrate can serve as the first region. Thus, thewide first region can be used for an epitaxial layer and the like withhigh properties.

Preferably, in the method of manufacturing a nitride substrate describedabove, in the step of cutting, the nitride substrate is cut such thatthe off angle has a maximum value at a second point in the secondregion, and the off angle monotonically decreases from the second pointto the first point.

Further, preferably, in the nitride substrate described above, the offangle has a maximum value at a second point in the second region, andthe off angle monotonically decreases from the second point to the firstpoint.

Thereby, since the first region located on the inner peripheral sidedoes not include the minimum value and the maximum value of the offangle, the off angle is not zero, and variations in the off angle can besuppressed. Thus, an epitaxial layer and the like with higher propertiescan be formed on the first region located on the inner peripheral side.

Preferably, in the method of manufacturing a nitride substrate describedabove, in the step of cutting, the nitride substrate is cut from thenitride crystal along a flat plane parallel to a flat plane inclinedfrom an a-plane or an m-plane in a c-axis direction. Further,preferably, in the nitride substrate described above, the front surfaceis inclined from an a-plane or an m-plane in a c-axis direction.

Thereby, properties of an epitaxial layer and the like produced usingthe nitride substrate can be further improved.

Preferably, the method of manufacturing a nitride substrate describedabove further includes the step of performing at least one of polishingand grinding of the front surface of the nitride substrate after thestep of cutting.

Thereby, the front surface of the nitride substrate can be flattened.Thus, an epitaxial layer and the like can be produced easily using thenitride substrate.

Preferably, in the method of manufacturing a nitride substrate describedabove, in the step of cutting, a plurality of the nitride substrates arecut. Thereby, manufacturing cost for each nitride substrate can bereduced.

Effects of the Invention

According to the method of manufacturing a nitride substrate and thenitride substrate of the present invention, by cutting the nitridesubstrate from the nitride crystal with the off angle being controlled,the nitride substrate can be manufactured with an improved yield suchthat the off angle formed between the axis orthogonal to the frontsurface and the m-axis or the a-axis is greater than zero.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view schematically showing a nitride substratein an embodiment of the present invention.

FIG. 2 is a schematic view showing an off angle when the nitridesubstrate in the embodiment of the present invention is seen from above(a front surface side).

FIG. 3 is a schematic view showing an off angle when the nitridesubstrate in the embodiment of the present invention is seen from above(the front surface side).

FIG. 4 is a schematic view showing an off angle when the nitridesubstrate in the embodiment of the present invention is seen from above(the front surface side).

FIG. 5 is a schematic view showing an off angle when the nitridesubstrate in the embodiment of the present invention is seen from above(the front surface side).

FIGS. 6(A) and 6(B) are cross sectional views schematically showing anitride crystal in the embodiment of the present invention.

FIG. 7 is a growth apparatus capable of being used to manufacture thenitride crystal in the embodiment of the present invention.

FIG. 8 is a schematic plan view of the nitride crystal in the presentembodiment seen from above.

FIG. 9 is a cross sectional view taken along a line IX-IX in FIG. 8, anda schematic view schematically showing a crystal orientation of thenitride crystal.

FIG. 10 is a cross sectional view taken along a line X-X in FIG. 8, anda schematic view schematically showing a crystal orientation of thenitride crystal.

FIG. 11 is a cross sectional view showing the nitride crystal in theembodiment of the present invention.

FIG. 12 is another cross sectional view showing the nitride crystal inthe embodiment of the present invention.

FIG. 13 is another cross sectional view showing the nitride crystal inthe embodiment of the present invention.

FIG. 14 is a schematic view showing an off angle when a nitridesubstrate obtained in a comparative example is seen from a directionvertical to a front surface.

FIG. 15 is a schematic view showing an off angle when the nitridesubstrate obtained in the comparative example is seen from the verticaldirection.

FIG. 16 is a schematic view showing an off angle when the nitridesubstrate obtained in the comparative example is seen from the verticaldirection.

FIG. 17 is a schematic view of a nitride crystal with a diameter 2Rhaving a front surface 22 a that can be approximated as a portion of anarc with a radius r and a center O in Example 1, seen from substantiallyan a-axis direction.

FIG. 18 is a schematic view of the nitride crystal with diameter 2Rhaving front surface 22 a that can be approximated as a portion of thearc with radius r and center O in Example 1, seen from substantially thea-axis direction.

FIG. 19 is a schematic view for further considering a sliced flat planeV2 in Example 1.

FIG. 20 is a schematic view for further considering sliced flat plane V2in Example 1.

FIG. 21 is a schematic view for further considering sliced flat plane V2in Example 1.

FIG. 22 is a schematic view for further considering sliced flat plane V2in Example 1.

FIG. 23 is a schematic view for further considering sliced flat plane V2in Example 1.

FIG. 24 is a schematic view for considering a sliced flat plane V3 inExample 1.

FIG. 25 is a schematic view for considering sliced flat plane V3 inExample 1.

FIG. 26 is a schematic view for considering a sliced flat plane V4 inExample 1.

FIG. 27 is a schematic view for illustrating a method of designating alocation for slicing a nitride crystal in Example 2.

MODES FOR CARRYING OUT THE INVENTION

Hereinafter, an embodiment of the present invention will be describedwith reference to the drawings. It is to be noted that, in the drawingsbelow, identical or corresponding parts will be designated by the samereference numerals, and the description thereof will not be repeated.Further, in the present specification, an individual orientation will beindicated by [ ], a group orientation will be indicated by < >, anindividual plane will be indicated by ( ), and a group plane will beindicated by { }. In addition, although a negative index iscrystallographically supposed to be indicated by placing “-” (a bar)above a numeral, it will be indicated in the present specification byplacing a minus sign before a numeral.

FIG. 1 is a perspective view schematically showing a nitride substratein the present embodiment. Firstly, referring to FIG. 1, the nitridesubstrate in the present embodiment will be described.

As shown in FIG. 1, a nitride substrate 10 includes a front surface 11.Front surface 11 has a first region 12 and a second region 13surrounding the first region 12. That is, the first region 12 is locatedon an inner peripheral side in front surface 11 of nitride substrate 10,and the second region 13 is located on an outer peripheral side in frontsurface 11 of nitride substrate 10. The first region 12 in the presentembodiment is a region in front surface 11 of nitride substrate 10 inwhich, of an epitaxial layer formed on front surface 11, an epitaxiallayer used for a substrate or a device is formed. Of the epitaxial layerformed on front surface 11, the second region 13 not used for asubstrate or a device has a distance t of, for example, within 2 mm froman edge.

FIGS. 2 to 5 are schematic views showing off angles when the nitridesubstrate in the present embodiment is seen from above (a front surfaceside). In FIGS. 2 to 5, an arrow is a vector indicating the magnitudeand direction of an off angle. Further, in FIGS. 2 to 5, a represents ana-axis direction, m represents an m-axis direction, and c represents anc-axis direction, indicating directions at a center of front surface 11of nitride substrate 10. As shown in FIGS. 2 to 5, an off angle formedbetween an axis orthogonal to front surface 11 and an m-axis or ana-axis is greater than zero in entire front surface 11. That is, frontsurface 11 does not include a region where the off angle is zero.

The off angle of front surface 11 may have the same magnitude as shownin FIGS. 2 to 4, or may have variations in magnitude as shown in FIG. 5.Further, the direction of the off angle may be always constant as shownin FIGS. 2 to 5, or may be different (not shown).

When the off angle of front surface 11 has variations in magnitude asshown in FIG. 5, the off angle has a minimum value at a first point 13 ain the second region 13. The off angle has a maximum value at a secondpoint 13 b in the second region 13. The off angle monotonicallydecreases from the second point 13 b to the first point 13 a. It is tobe noted that monotonic decrease means that the magnitude of the offangle is always identical or decreases from the second point 13 b to thefirst point 13 a, and the off angle at the first point 13 a is smallerthan the off angle at the second point 13 b. That is, monotonic decreasedoes not include a portion in which the off angle increases from thesecond point 13 b to the first point 13 a.

Further, front surface 11 is inclined from an a-plane or an m-plane inthe c-axis direction. It is to be noted that a c-plane refers to a{0001} plane, including a (0001) plane, a (000-1) plane, and planesparallel thereto. The m-axis direction refers to a <1-100> direction,including a [1-100] direction, a [10-10] direction, a [−1100] direction,a [−1010] direction, a [01-10] direction, and a [0-110] direction.Further, the a-axis direction refers to a<11-20> direction, including a[11-20] direction, a [1-210] direction, a [−2110] direction, a [−1-120]direction, a [−12-10] direction, and a [2-1-10] direction.

The off angle in at least the first region 12 of front surface 11 ispreferably not less than 0.15° and less than 2°, and more preferably notless than 0.3° and less than 0.7°. Within this range, an epitaxial layerwith high properties can be formed on at least the first region 12.

Nitride substrate 10 of the present embodiment has front surface 11 inthe shape of a rectangular plate. If front surface 11 is rectangular, itis preferable that a maximum value of a distance between one point andanother point on the edge of front surface 11 is not less than 5 mm. Iffront surface 11 is circular or elliptical, it is preferable that thelongest diameter is not less than 10 mm.

Nitride substrate 10 is, for example, In_(x)Al_(y)Ga_((1-x-y)) N (0≦x≦1,0≦y≦1, 0≦x+y≦1), and preferably gallium nitride (GaN), AlN, AlGaN, orthe like.

Next, a method of manufacturing the nitride substrate in the presentembodiment will be described.

FIGS. 6(A) and 6(B) are cross sectional views schematically showing anitride crystal in the present embodiment. FIG. 7 is a growth apparatuscapable of being used to manufacture the nitride crystal in the presentembodiment. As shown in FIGS. 6(A), 6(B), and 7, firstly, a nitridecrystal 22 is grown. Nitride crystal 22 is an ingot for manufacturingnitride substrate 10. In the present embodiment, nitride crystal 22 isgrown, for example, by a sublimation method.

Here, main components of a growth apparatus 100 in the presentembodiment will be described with reference to FIG. 7. Growth apparatus100 is an apparatus for growing a crystal by the sublimation method.

As shown in FIG. 7, growth apparatus 100 mainly includes a crucible 101,a heat body 121, a reaction vessel 123, and a heating portion 125.

Crucible 101 is made of, for example, graphite. Crucible 101 has an airexhaust outlet 101 a. Heat body 121 is provided around crucible 101 in amanner ensuring ventilation between an interior and an exterior ofcrucible 101. Reaction vessel 123 is provided around heat body 121.Heating portion 125 such as a high frequency heating coil for heatingheat body 121 is provided at an outer central portion of reaction vessel123.

At one ends of heat body 121 and reaction vessel 123, inlets 121 a, 123a for allowing a carrier gas such as nitrogen gas to flow into crucible101 disposed within reaction vessel 123, and outlets 121 b, 123 b foremitting the carrier gas to an exterior of reaction vessel 123 areprovided. Further, above and below reaction vessel 123, radiationthermometers 127 a, 127 b for measuring temperatures above and belowcrucible 101 are provided.

Although growth apparatus 100 described above may include variouselements other than those described above, for convenience ofexplanation, these elements will not be shown and described.

Firstly, a base substrate 21 is prepared. The base substrate is notparticularly limited, and may be a substrate of a different type such asan SiC (silicon carbide) substrate, or may be made of a materialidentical to that of nitride crystal 22 to be grown. In the presentembodiment, for example, an SiC substrate having a (0001) plane as amain surface is prepared as base substrate 21. Base substrate 21 isplaced at an upper portion of crucible 101. On this occasion, a frontsurface of base substrate 21 is flattened, and a base substrateprotection material made of, for example, graphite is placed on a rearsurface side to closely adhere thereto to suppress sublimation of basesubstrate 21.

Thereafter, a raw material 17 is prepared. If an MN crystal is grown asnitride crystal 22, for example, an MN powder or the like is used as rawmaterial 17. Raw material 17 is placed at a lower portion of crucible101 to face base substrate 21 with each other.

Thereafter, the temperature within crucible 101 is increased by heatingheat body 121 using heating portion 125 while causing the nitrogen gasto flow into reaction vessel 123. Then, raw material 17 is heated up toa temperature at which raw material 17 sublimates. Through the heating,raw material 17 sublimates and generates a sublimation gas. Thesublimation gas is solidified again on the front surface of basesubstrate 21 set at a temperature lower than that of raw material 17. Inthe present embodiment, heating is performed such that, for example,base substrate 21 has a temperature of 2000° C. and raw material 17 hasa temperature of 2200° C., to grow a nitride crystal having a thicknessof 30 urn, and the nitride crystal is further grown for 100 hours.Thereby, a nitride crystal having a thickness of, for example, 10 mm canbe grown. Thereafter, the nitride crystal is cooled down to a roomtemperature (for example, 25° C.), and taken out from growth apparatus100. Thereby, the nitride crystal can be grown on base substrate 21.Thereafter, raw material 17 is further replenished to further grow anitride crystal on the nitride crystal. Thereby, as shown in FIG. 6(A)or 6(B), nitride crystal 22 having a thickness of, for example, 19 mm isobtained. A front surface 22 a of nitride crystal 22 is warped in aconcave shape. Further, a rear surface 22 b of nitride crystal 22 may bewarped. Warpages of front surface 22 a and rear surface 22 b may beidentical as shown in FIG. 6(B), or may be different as shown in FIG.6(A). It is to be noted that, as shown in FIG. 6(B), base substrate 21may sublimate due to the growth of nitride crystal 22.

Although the sublimation method is employed as a method of growingnitride crystal 22 in the present embodiment, the method of growingnitride crystal 22 is not particularly limited to the sublimationmethod, and, for example, a vapor deposition method such an HVPE(Hydride Vapor Phase Epitaxy) method, an MBE (Molecular Beam Epitaxy)method, and an MOCVD (Metal Organic Chemical Vapor Deposition) method, aflux method, and a liquid phase method such as a high nitrogen pressuresolution method can be employed.

For example, when nitride crystal 22 is grown by the HVPE method, it isgrown, for example, as described below. Firstly, base substrate 21 isprepared. As base substrate 21, for example, a GaAs (gallium arsenide)substrate having a (111) plane as a main surface is used. Thereafter, amask is formed on base substrate 21, and nitride crystal 22 is grown bythe HVPE method. As nitride crystal 22, a GaN crystal having a thicknessof, for example, 10 mm is grown. Thereafter, the base substrate isremoved by etching with, for example, aqua regia. Thereby, nitridecrystal 22 shown in FIG. 6(B) is obtained.

FIG. 8 is a schematic plan view of the nitride crystal in the presentembodiment seen from above. FIGS. 9 and 10 are cross sectional viewstaken along a line IX-IX and a line X-X in FIG. 8, and schematic viewsschematically showing crystal orientations of the nitride crystal. InFIGS. 9 and 10, a dashed line indicates the c-axis direction, the a-axisdirection, or the m-axis direction at each position. Front surface 22 aof nitride crystal 22 grown as described above is warped in a concaveshape as shown in FIGS. 6(A), 6(B), 9, and 10. In the presentembodiment, since nitride crystal 22 is grown in the c-axis direction,or front surface 22 a, rear surface 22 b, or an inside lattice plane ofnitride crystal 22 is warped, an orientation of the c-axis, the a-axis,or the m-axis is different depending on a position in nitride crystal22.

Since nitride substrate 10 is cut from nitride crystal 22 in the presentembodiment, nitride crystal 22 having a large thickness is grown as aningot. When nitride crystal 22 having a large thickness is grown, frontsurface 22 a of nitride crystal 22 is warped. Therefore, an off angleformed between front surface 22 a of nitride crystal 22 and the a-axisor the m-axis varies depending on a position in front surface 22 a. Thesame applies to rear surface 22 b.

Further, difference in the orientation of the c-axis, the m-axis, or thea-axis for each position in front surface 22 a of nitride crystal 22 ismeasured, for example, by an X-ray Diffraction (XRD) method. Measurementis similarly performed in rear surface 22 b. In addition, measurement issimilarly performed for the inside of nitride crystal 22, by exposing alocation thereof.

Next, nitride substrate 10 including front surface 11 having the firstregion 12 and the second region 13 surrounding the first region 12 iscut from nitride crystal 22. In the step of cutting, nitride substrate10 is cut such that the off angle formed between the axis orthogonal tofront surface 11 and the m-axis or the a-axis is greater than zero.

In the step of cutting, it is preferable that nitride substrate 10 iscut such that the off angle formed between the axis orthogonal to frontsurface 11 and the m-axis or the a-axis has a minimum value at the firstpoint 13 a in the second region 13. Further, it is preferable thatnitride substrate 10 is cut such that the second region 13 is within 2mm from an edge of nitride substrate 10. Furthermore, in the step ofcutting, it is preferable that nitride substrate 10 is cut such that theoff angle has a maximum value at the second point in the second region13, and the off angle monotonically decreases from the second point 13 bto the first point 13 a.

Here, the step of cutting will be described more specifically. FIG. 11is a cross sectional view showing nitride crystal 22 in the presentembodiment. In FIG. 11, a represents the a-axis direction, m representsthe m-axis direction, and c represents the c-axis direction, indicatingdirections of the axes at a center of nitride crystal 22. In a casewhere it can be considered that front surface 22 a and rear surface 22 bof nitride crystal 22 have the same radius of curvature, nitridesubstrates 10 are cut from nitride crystal 22 along flat planes T1, T2parallel to the m-plane at a center of front surface 22 a as shown forexample in FIG. 11. Flat planes T1, T2 are not orthogonal to the m-axisof front surface 22 a and rear surface 22 b of nitride crystal 22. Frontsurfaces 11 of nitride substrates 10 cut along flat planes T1, T2 haveoff angles as shown in FIGS. 2 and 3, respectively. Since flat plane T2has an inclination from the m-plane in the c-axis direction greater thanthat of flat plane T1, flat plane T2 has a greater off angle.

FIG. 12 is another cross sectional view showing nitride crystal 22 inthe present embodiment. In FIG. 12, a represents the a-axis direction, mrepresents the m-axis direction, and c represents the c-axis direction,indicating the axes at the center of nitride crystal 22. In the casewhere it can be considered that front surface 22 a and rear surface 22 bof nitride crystal 22 have the same radius of curvature, nitridesubstrates 10 are cut from nitride crystal 22 along flat planes U1, U2parallel to a flat plane inclined from the m-plane at the center offront surface 22 a in the c-axis direction as shown for example in FIG.12. Flat planes U1, U2 are planes inclined from a direction in which thenitride crystal is grown (i.e., the c-axis direction at the center). Inthe present embodiment, flat planes U1,U2 are inclined by 0.2° from aflat plane vertical to the c-plane at the center of front surface 22 aof nitride crystal 22. Flat planes U1, U2 are not orthogonal to them-axis of front surface 22 a and rear surface 22 b of nitride crystal22. Front surfaces 11 of nitride substrates 10 cut along flat planes U1,U2 have off angles as shown in FIGS. 4 and 3, respectively, and theorientations of the off angles are opposite to each other.

FIG. 13 is another cross sectional view showing nitride crystal 22 inthe present embodiment. In FIG. 13, a represents the a-axis direction, mrepresents the m-axis direction, and c represents the c-axis direction,indicating directions of the axes at the center of front surface 22 a ofnitride crystal 22. As shown in FIG. 13, in a case where nitride crystal22 is grown in the c-axis direction, nitride substrate 10 is cut fromnitride crystal 22 along a flat plane W1 which passes through frontsurface 22 a and rear surface 22 b of nitride crystal 22 and does notpass through centers of the radii of curvature of front surface 22 a andrear surface 22 b of nitride crystal 22 (a center O1 and a center O2)and therebetween (a line segment connecting center O1 with center O2).That is, flat plane W1 is not orthogonal to the m-axis of front surface22 a and rear surface 22 b of nitride crystal 22. In further otherwords, flat plane W1 is not located between center O1 of the radius ofcurvature of front surface 22 a and center O2 of the radius of curvatureof rear surface 22 b of nitride crystal 22. Namely, flat plane W1 is notorthogonal to the m-axis in entire nitride crystal 22. Front surface 11of nitride substrate 10 cut along flat plane W1 has an off angle asshown in FIG. 5.

Here, the “radii of curvature of front surface 22 a and rear surface 22b” mean radii when curves of front surface 22 a and rear surface 22 b ofnitride crystal 22 are approximated as arcs. In addition, the “center ofthe radius of curvature” means a center of the above approximated arc.

Further, based on the crystal orientation of nitride crystal 22 measuredfor example by the XRD method, nitride substrate 10 can be cut fromnitride crystal 22 as shown in FIGS. 11 to 13.

Preferably, nitride substrate 10 is cut from nitride crystal 22 along aflat plane parallel to a flat plane inclined from the a-plane or them-plane in the c-axis direction, such as flat planes U1, U2, W1.Particularly preferably, nitride substrate 10 is cut from nitridecrystal 22 along a flat plane parallel to a flat plane inclined from thea-plane or the m-plane located at the center of nitride crystal 22 inthe c-axis direction. In a case where front surface 22 a of nitridecrystal 22 has a polygonal shape, the center of nitride crystal 22 meansa center of a maximum length of diameters of circles inscribed inarbitrarily specified opposing two sides sandwiching a central portionin front surface 22 a. In a case where front surface 22 a of nitridecrystal 22 has a circular or elliptical shape, the center of nitridecrystal 22 means a center of a maximum length of diameters arbitrarilyspecified in front surface 22 a.

In the step of cutting, it is preferable to cut a plurality of nitridesubstrates 10. If nitride crystal 22 has a size of, for example, notless than 10 mm, the plurality of nitride substrates 10 can be easilycut.

A method of cutting nitride substrate 10 is not particularly limited,and for example, a mechanical method such as slicing can be used.Slicing refers to mechanically cutting nitride substrate 10 from nitridecrystal 22 with a slicer having an outer peripheral cutting edge, aslicer having an inner peripheral cutting edge, a wire saw, or the like.

Nitride substrate 10 manufactured as described above does not include aregion where the off angle formed between the axis orthogonal to thefront surface and the a-axis or the m-axis is zero.

Next, at least one of polishing and grinding of the front surface ofnitride substrate 10 is performed as necessary. Grinding refers togrinding down the front surface in a thickness direction by bringing arotating grindstone to be in contact with the front surface. Sinceparticle detachment can be suppressed in nitride substrate 10 duringpolishing and grinding, front surface 11 can be easily flattened. It isto be noted that at least one of polishing and grinding of the rearsurface of nitride substrate 10 may further be performed.

In the present embodiment, nitride substrate 10 is shaped using agrindstone having diamond abrasive grains fixed thereto, and thereafterfront surface 11 of nitride substrate 10 is ground or polished usingdiamond abrasive grains.

By performing the above steps, a plurality of nitride substrates 10having a thickness of, for example, 400 μm to 450 μm can bemanufactured.

Next, an effect of nitride substrate 10 manufactured by the method ofmanufacturing nitride substrate 10 in the present embodiment will bedescribed.

In the present embodiment, nitride substrates 10 are cut from nitridecrystal 22 along flat planes T1, T2 of FIG. 11 and flat planes U1, U2 ofFIG. 12 not orthogonal to the m-axis and the a-axis of front surface 22a and rear surface 22 b. Thereby, nitride substrates 10 in which the offangle formed between the axis orthogonal to the front surface and them-axis or the a-axis is greater than zero as shown in FIGS. 2, 3, 4, and3, respectively, can be manufactured.

On the other hand, in a comparative example, nitride substrates 50 (seeFIG. 14) are cut from nitride crystal 22 along a flat plane T3 of FIG.11 and a flat plane U3 of FIG. 12 in which at least one of front surface22 a and rear surface 22 b is orthogonal to the m-axis or the a-axis.Since both front surface 22 a and rear surface 22 b are orthogonal tothe m-axis or the a-axis in the case of the comparative example, an offangle is zero in an entire front surface 51 of nitride substrate 50 asshown in FIG. 14. FIG. 14 is a schematic view showing an off angle whenthe nitride substrate obtained in the comparative example is seen from adirection vertical to the front surface. In FIG. 14, a represents thea-axis direction, m represents the m-axis direction, and c representsthe c-axis direction, indicating directions at a center of front surface51 of nitride substrate 50. No arrows are illustrated in FIG. 14 becausethe magnitude of the vector indicating the off angle is zero in theentire front surface 51 of nitride substrate 50.

An epitaxial layer is formed on each of front surface 11 of nitridesubstrate 10 in the present embodiment and front surface 51 of nitridesubstrate 50 in the comparative example manufactured as described above.Since the epitaxial layer formed on a region having an off angle iscrystal-grown in a lateral direction, the epitaxial layer grown on frontsurface 11 of nitride substrate 10 has good front surface morphology.That is, an epitaxial layer with high properties can be formed onnitride substrate 10 having front surface 11 in which the off angle isformed entirely as in the present embodiment. Thus, properties of asubstrate, a device, and the like fabricated using the epitaxial layercan be improved in a wide range. Therefore, nitride substrate 10 in thepresent embodiment can control the off angle such that properties of theepitaxial layer formed on front surface 11 and a device using the sameare improved.

On the other hand, an epitaxial layer with high properties cannot beformed on a region where an off angle is zero. Therefore, the epitaxiallayer grown on front surface 51 of nitride substrate 50 in thecomparative example cannot obtain good morphology. That is, an epitaxiallayer with high properties cannot be formed on nitride substrate 50having front surface 51 in which the off angle is not formed entirely asin the comparative example. Thus, it is not possible to improveproperties of a substrate, a device, and the like fabricated using theepitaxial layer. Therefore, nitride substrate 50 in the comparativeexample cannot control the off angle such that properties of theepitaxial layer formed on front surface 51 and a device using the sameare improved.

Further, as a result of earnestly studying the position and angle atwhich nitride substrate 10 is cut from nitride crystal 22, the inventorsof the present invention have found that, when the nitride substratesare cut along flat planes W2, W3 which pass through centers O1, O2 ofthe radii of curvature of front surface 22 a and rear surface 22 b ofnitride crystal 22 as shown in FIG. 13, the front surfaces of the cutnitride substrates have an off angle of zero. Thus, in the presentembodiment, nitride substrate 10 is cut from nitride crystal 22 alongflat plane W1 which passes through front surface 22 a and rear surface22 b of nitride crystal 22 and does not pass through centers O1, O2 ofthe radii of curvature of front surface 22 a and rear surface 22 b ofnitride crystal 22 (flat plane W1 which does not pass through the linesegment connecting centers O1 and O2 of the radii of curvature) as shownin FIG. 13. Thereby, nitride substrate 10 in which the off angle formedbetween the axis orthogonal to the front surface and the m-axis or thea-axis is greater than zero as shown in FIG. 5 can be manufactured.

Further, as the comparative example, nitride substrates 50 are cut fromnitride crystal 22 along flat planes W2, W3 which pass through frontsurface 22 a and rear surface 22 b of nitride crystal 22 and passthrough centers O1, O2 of the radii of curvature of front surface 22 aand rear surface 22 b of nitride crystal 22. Thereby, nitride substrates50 each including a second region 53 having a first point 53 a at whichan off angle formed between an axis orthogonal to front surface 51 andthe m-axis or the a-axis is zero, and a first region 52 located on aninner peripheral side of the second region 53 are manufactured as shownin FIGS. 15 and 16. FIGS. 15 and 16 are schematic views showing offangles when the nitride substrates obtained in the comparative exampleare seen from the direction vertical to the front surface. In FIGS. 15and 16, an arrow is a vector indicating the magnitude and direction ofan off angle. In FIGS. 15 and 16, a represents the a-axis direction, mrepresents the m-axis direction, and c represents the c-axis direction,indicating directions at the center of front surface 51 of nitridesubstrate 50.

Further, in a case where front surface 22 a and rear surface 22 b ofnitride crystal 22 have different radii of curvature, nitride substrate10 having an off angle with a minimum value at the first point 13 a inthe second region 13 can be manufactured when nitride substrate 10 iscut from nitride crystal 22 along flat plane W1 which does not pass theline segment connecting centers O1 and O2 of the radii of curvature offront surface 22 a and rear surface 22 b of nitride crystal 22 as shownin FIG. 13.

The epitaxial layer formed on the first region 12 located on the innerperipheral side on front surface 11 of nitride substrate 10 is used fora substrate, a device, and the like. Thus, in the present embodiment,the off angle of nitride substrate 10 is controlled such that the offangle in the second region 13 of nitride substrate 10 as a region notsubstantially used for a substrate, a device, and the like has a minimumvalue. Therefore, when an epitaxial layer is formed using nitridesubstrate 10 and used for a substrate, a device, and the like, the offangle can be controlled to further improve properties of a region to beused, that is, the first region 12.

In addition, in the present embodiment, nitride substrate 10 is cut fromnitride crystal 22 to have front surface 11 in which the off angle iscontrolled. Thus, nitride substrate 10 having a controlled off angle canbe manufactured stably irrespective of the state of nitride crystal 22(that is, without depending on growth conditions and the like of nitridecrystal 22). Therefore, nitride substrate 10 in which the off angle iscontrolled to form an epitaxial layer with high properties can bemanufactured with an improved yield.

When nitride crystal 22 is grown in the c-axis direction, nitridecrystal 22 is generally warped in a concave shape with respect to thec-axis direction. In the present embodiment, in view of the warpedshape, nitride substrate 10 is cut to control distribution of the offangle formed between the axis orthogonal to the front surface and them-axis or the a-axis. Thus, nitride substrate 10 in which the off angleformed between the axis orthogonal to the front surface and the m-axisor the a-axis of the front surface is controlled can be manufacturedwith an improved yield.

Example 1

In the present example, a slicing method in the step of cutting thenitride substrate from the nitride crystal such that the off angleformed between the axis orthogonal to the front surface and the m-axisor the a-axis is greater than zero was considered.

(Slicing Method 1)

FIGS. 17 and 18 are schematic views of a nitride crystal with a diameter2R having front surface 22 a that can be approximated as a portion of anarc with a radius r and a center O (i.e., with r as a radius ofcurvature and O as the center of the radius of curvature), seen fromsubstantially the a-axis direction. Here, the thickness of the nitridecrystal will be ignored for brevity.

Firstly, a method of cutting a nitride substrate in which an off anglefrom the m-plane is zero in the example shown in FIG. 17 will beconsidered. In this case, the nitride substrate is obtained, forexample, by slicing a nitride crystal along a flat plane vertical tofront surface 22 a of the nitride crystal as represented by a flat planeV1. As a result of generalization, any plane having a portion common tothe nitride crystal, of planes as represented by straight lines passingthrough center O, falls under the above flat plane.

Next, a method of cutting a nitride substrate in which an off angle fromthe m-plane is β in the example shown in FIG. 18 will be considered. Inthis case, as is clear from the result of FIG. 17, the nitride substratecan be obtained by slicing the nitride crystal along a flat plane asrepresented by a flat plane V2 slightly deviated from center O.

FIGS. 19 to 21 are schematic views for further considering sliced flatplane V2. Although the shape of the nitride crystal has been consideredas a portion of the arc with radius r and center O in the example shownin FIG. 17 and in the example shown in FIG. 18, the shape of the nitridecrystal (front surface 22 a) will be considered as an entire arc asshown in FIGS. 20 and 21. In this case, it can be seen that any planenot passing through a region with a radius ρ (ρ=r×sin β) around center O(any plane passing through a region A in FIGS. 20 and 21) falls undersliced flat plane V2 for cutting the nitride substrate in which the offangle from the m-plane is β. In other words, any plane with a distanceof ρ from center O falls under flat plane V2. The distance from center Omeans a length of a perpendicular extending from center O to any plane.

However, since the nitride crystal is actually merely a portion of thearc with radius r and center O, flat plane V2 along which the nitridecrystal can be sliced is a portion thereof. FIGS. 22 and 23 show thisstate. It can be seen that flat plane V2 along which the nitridesubstrate actually having an off angle of β can be sliced from thenitride crystal is in the portion of a region B in FIGS. 22 and 23.Here, for brevity, only a case where off angle β is placed on the leftside of a normal on front surface 22 a of the nitride crystal is shown,and a case where off angle β is placed on the right side thereof willnot be provided.

Therefore, based on the above consideration, it has been found that flatplane V2 for cutting the nitride substrate in which the off angle fromthe m-plane is β has the following two conditions:

(1) flat plane V2 has a distance of ρ from center O; and

(2) flat plane V2 has a portion common to front surface 22 a of thenitride crystal.

From the foregoing, it has been found that sliced flat plane V2 forobtaining the nitride substrate in which the off angle is β can bereadily determined and sliced by measuring the position of center O,radius of curvature r, and diameter 2R of the nitride crystal in thecase where front surface 22 a of the nitride crystal is approximated asan arc, with X-ray diffraction, a ruler, a caliper, and the like,respectively.

(Slicing Method 2)

A method of cutting a nitride substrate in which an off angle from them-plane is in a range of β1 to β2 (β1<β2) will be considered withreference to FIGS. 24 and 25. FIGS. 24 and 25 are schematic views forconsidering a sliced flat plane V3.

Based on the consideration similar to that in Slicing Method 1 describedabove, it has been found that flat plane V3 for cutting the nitridesubstrate in which the off angle from the m-plane is β1 to β2 has thefollowing two conditions:

(1) in flat plane V3, distance ρ from center O satisfies r×sinβ1<ρ<r×sin β2 (a region C in FIGS. 24 and 25); and

(2) flat plane V3 has a portion common to front surface 22 a of thenitride crystal.

From the foregoing, it has been found that sliced flat plane V3 forobtaining the nitride substrate in which the off angle is β1 to β2 canbe readily determined and sliced by measuring the position of center O,radius of curvature r, and diameter 2R of the nitride crystal in thecase where front surface 22 a of the nitride crystal is approximated asan arc, with X-ray diffraction, a ruler, a caliper, and the like,respectively.

(Slicing Method 3)

An effect of the thickness of the nitride crystal ignored for brevity inSlicing Methods 1 and 2 described above will be considered withreference to FIG. 26. FIG. 26 is a schematic view for considering asliced flat plane V4.

When front surface 22 a and rear surface 22 b of a prepared nitridecrystal are measured by X-ray diffraction, it is found that the nitridecrystal has front surface 22 a which can be approximated as a portion ofan arc with a radius r1 and center O1, and rear surface 22 b which canbe approximated as a portion of an arc with a radius r2 and center O2.Based on the foregoing discussion, it is only necessary to considerspherical surfaces with radii of r1×sin β1, r1×sin β2 determineddepending on the range of the off angle from β1 to β2 (β1<β2) aroundcenter O1, and also consider spherical surfaces with radii of r2×sin β1,r2×sin β2 around center O2.

Therefore, it has been found that flat plane V4 for cutting a nitridesubstrate in which an off angle from the m-plane is β1 to β2 has thefollowing three conditions:

(1) in flat plane V4, distance ρ from center O1 satisfies r1×sinβ1<ρ<r1×sin β2 (a region D1 in FIG. 26);

(2) in flat plane V4, distance ρ from center O2 satisfies r2×sinβ1<ρ<r2×sin β2 (a region D2 in FIG. 26); and

(3) flat plane V4 has a portion common to front surface 22 a and rearsurface 22 b of the nitride crystal.

From the foregoing, it has been found that sliced flat plane V4 forobtaining the nitride substrate in which the off angle is β1 to β2 canbe readily determined and sliced by measuring the positions of centersO1, O2, radii of curvature r1, r2, and diameter 2R of the nitridecrystal in the case where front surface 22 a and rear surface 22 b ofthe nitride crystal are approximated as arcs, with X-ray diffraction, aruler, a caliper, and the like, respectively.

Since the change in the radius of curvature in a growth thicknessdirection of the nitride crystal is considered as monotonic change in agood crystal, slicing can be performed with accuracy merely by focusingattention on the front surface and the rear surface as considered above.

Further, the c-plane other than the front surface and the rear surfaceof the nitride crystal can also be considered by increasing an arc witha center and a radius of curvature. As an effect of considering in thatmanner, the change in the radius of curvature in the growth thicknessdirection can be grasped more correctly, and distribution of the offangle within the sliced nitride substrate can be further improved. Forexample, in a case where nitride crystal 22 does not have good quality,or where nitride crystal 22 is grown in the c-axis direction using asubstrate having an off angle as base substrate 21, a sliced plane forobtaining a substrate having a controlled off angle can be determinedwith more accuracy by increasing an arc with a center and a radius ofcurvature for the c-plane other than the front surface and the rearsurface and performing similar consideration. The center and the radiusof curvature in the c-plane inside nitride crystal 22 can be measured bythe X-ray Diffraction (XRD) method, for example by exposing a locationthereof.

Although the methods of slicing a nitride substrate having an off anglewith respect to the m-plane have been considered in Slicing Methods 1 to3 described above, Slicing Methods 1 to 3 can also be applied to amethod of slicing a nitride substrate having an off angle with respectto the a-plane. That is, Slicing Methods 1 to 3 can be applied to aslicing method cutting a nitride substrate such that the off angleformed between the axis orthogonal to the front surface and the m-axisor the a-axis is greater than zero.

A method of cutting a nitride substrate such that, with regard to thefirst region included in the front surface of the nitride substrate andthe second region surrounding the first region, the second region iswithin 2 mm from the edge can also be considered similarly. That is, ifthe nitride substrate is sliced with coordinates of slicing points 1 and2 specified in Example 2 described later being corrected, the nitridesubstrate can be cut such that the second region is within 2 mm from theedge.

Example 2

In the present example, a concrete slicing method for cutting a nitridesubstrate from a grown nitride crystal such that the off angle formedbetween the axis orthogonal to the front surface and the m-axis or thea-axis is greater than zero was considered. To consider the slicingmethod, a coordinate system fixed to a nitride crystal shown in FIG. 27was defined to specify a slicing position.

FIG. 27 is a schematic view for illustrating a method of designating alocation for slicing a nitride crystal in the present example. As shownin FIG. 27, a coordinate origin was set at a center of rear surface 22 bof the nitride crystal, a y-axis direction was set along the c-axis(i.e., the axis of rotational symmetry of the crystal), and a z-axisdirection was set in a direction in which a wire of a wire sawreciprocates. When a nitride substrate with the m-plane (the a-plane)having an off angle was obtained, the a-axis (the m-axis) was orientedfrontward (a z-direction). Slicing was performed using the wire to passthrough two slicing points (slicing points 1, 2) designated by thecoordinate system. The coordinates of slicing points 1 and 2 were set to(x1,y1), (x2,y2), respectively. In addition, since the wire moved in thevertical direction, the nitride crystal was inclined by a jig (notshown) such that a sliced plane was vertical.

Cases 1 to 3 of the present invention and Comparative Example 1 belowdescribe the details. Table 1 below describes manufacturing conditionsand results of manufactured nitride substrates in Cases 1 to 3 of thepresent invention and Comparative Example 1.

TABLE 1 Case 1 of Case 2 of Case 3 of Com- Present Present Presentparative Invention Invention Invention Example 1 Nitride single crystalAIN GaN GaN AIN Radius of curvature on 1.0 5.0 11.0 1.0 front surfaceside (m) Radius of curvature on 1.1 5.5 10.5 1.1 rear surface side (m)Radius of crystal (mm) 25 37.5 50 25 Thickness at center of 10 20 35 10crystal (mm) Coordinate (x,y) of  (8,1010) (19,5020) (211,11035)(0,1010) slicing point 1 (mm) Coordinate (x,y) of (10,1100) (22,5500)(202,10500) (0,1100) slicing point 2 (mm) Substantial plane m m a morientation of sliced substrate Number of sliced 1 2 2 2 substratesobtained (number) Grinding/polishing Performed Not Performed Performedstep performed Range of off angle 0.4 to 0.6 0.15 to 0.3 1 to 1.2 0(deg)

(Case 1 of Present Invention)

A 2-inch SiC substrate having a (0001) plane as a main surface wasprepared, and an AlN single crystal was grown on the substrate as basesubstrate 21 by the sublimation method.

Specifically, base substrate 21 was placed at the upper portion ofcrucible 101 shown in FIG. 6. On this occasion, a front surface of basesubstrate 21 was flattened, and a base substrate protection materialmade of graphite was placed on a rear surface side to closely adherethereto to suppress sublimation of base substrate 21. Further, an AlNpowder raw material was prepared, and placed as raw material 17 at thelower portion of crucible 101 to face base substrate 21 with each other.

Thereafter, the temperature within crucible 101 was increased by heatingheat body 121 using heating portion 125 while causing the nitrogen gasto flow into reaction vessel 123. Heating was performed such that basesubstrate 21 had a temperature of 2000° C. and raw material 17 had atemperature of 2200° C., to grow an AlN single crystal having athickness of 30 and the MN single crystal was further grown for 100hours. Thereafter, the MN single crystal was cooled down to a roomtemperature, and the SiC substrate was removed. Thereby, an MN singlecrystal as nitride crystal 22 having a growth thickness of about 10 mmin the c-axis direction was obtained.

When crystallinity in both a front surface and a rear surface of theobtained MN single crystal was evaluated using X-ray diffraction, it wasfound that the MN single crystal was a good single crystal having ahalf-value width of about 100 seconds. It was found from a result ofmeasurement of (0002) rocking curves at many points that the singlecrystal was warped in a rotationally symmetric shape that was upwardlyconcave with the c-axis direction being oriented upward. The frontsurface had a radius of curvature of 1.0 m, and the rear surface had aradius of curvature of 1.1 m. It was confirmed that the shape of thec-plane can be approximated as an arc. It was confirmed that the singlecrystal was in a state as schematically shown in FIGS. 26 and 27.

The MN single crystal was set on a wire saw with a plane orientationthereof being checked, and sliced along a flat plane passing throughslicing point 1 and slicing point 2 indicated in Table 1. Thereby, oneMN substrate was obtained.

Thereafter, grinding was performed using a grindstone containing diamondabrasive grains, and polishing was performed using slurry containingdiamond abrasive grains. Thereby, an MN substrate of Case 1 of thepresent invention was manufactured. Through grinding and polishing, afront surface of the nitride substrate of Case 1 of the presentinvention was able to be flattened. Therefore, an epitaxial layer wasable to be fabricated more easily using the nitride substrate of Case 1of the present invention, when compared with a nitride substrate of Case2 of the present invention not subjected to grinding and polishingdescribed later.

When measurement of in-plane mapping of an off angle was performed onthe manufactured AlN substrate by X-ray diffraction to examine the rangeof distribution, an AlN substrate in which a front surface wassubstantially the m-plane and did not include a region where the offangle was zero, and which had an off angle particularly preferable forepitaxial growth in the entire region of the front surface was obtained.

(Case 2 of Present Invention)

A 3-inch GaAs substrate having a (111) plane as a main surface wasprepared, and an entire front surface of the GaAs substrate was coveredwith a thin mask. SiO₂ (silicon dioxide) was used as a material for themask, as it has a property that does not allow GaN to grow directlythereon. A window was formed in the mask, and GaN was epitaxially grownthrough the window by the HVPE method.

In an HVPE furnace used for the growth, a Ga boat was provided at anupper portion inside the vertically long furnace, and a Ga melt was heldtherein. A susceptor was provided at a lower portion of the furnace, andthe GaAs substrate was set thereon. A heater was provided around thefurnace to heat the furnace. A mixed gas containing hydrogen gas and HClgas was introduced from a gas inlet. HCl reacted with Ga to synthesizeGaCl, and GaCI in the form of a gas flew downward. A mixed gascontaining hydrogen gas and NH₃ gas was introduced from the gas inlet.GaCI reacted with NH₃ to synthesize GaN, and GaN was deposited on theGaAs substrate. A buffer layer was firstly grown at a low temperature(490° C.), and thereafter the temperature was increased and epitaxialgrowth was performed at a high temperature (1010° C.). Thus, a GaNsingle crystal having a thickness of 20 mm was grown as nitride crystal22. Thereafter, the base substrate was removed by etching with aquaregia. Thereby, a GaN single crystal as nitride crystal 22 shown in FIG.6(B) was obtained.

When crystallinity in both a front surface and a rear surface of theobtained GaN single crystal was evaluated as in Case 1 of the presentinvention, it was found that the GaN single crystal had a half-valuewidth of about 100 seconds. In addition, warped shapes were confirmedusing X-ray diffraction (see Table 1).

Base on that, wire slicing was performed along a flat plane passingthrough slicing points 1, 2 described in Table 1 to obtain two GaNsubstrates, and thus a GaN substrate of Case 2 of the present inventionwas manufactured. Since grinding and polishing were not performed afterthe slicing in Case 2 of the present invention, a front surface was notable to be flattened when compared with that in Case 1 of the presentinvention. However, an epitaxial layer was also able to be formed usingthe nitride substrate.

When distribution of an off angle was evaluated for the manufactured GaNsubstrate, it was confirmed that the GaN substrate had a front surfacewhich was substantially the m-plane and did not include an off angle ofzero.

(Case 3 of Present invention)

A GaN single crystal grown in an HVPE furnace similar to that in Case 2of the present invention was prepared. Similarly, it was confirmed thatcrystallinity in both a front surface and a rear surface of the obtainedGaN single crystal was about 100 seconds, and warped shapes were alsoconfirmed, using X-ray diffraction (see Table 1).

Based on that, in this case, to obtain a substrate with substantiallythe a-plane, the GaN single crystal was set on a wire saw with adirection thereof being changed from that in Case 2 of the presentinvention. Specifically, wire slicing was performed along a flat planepassing through slicing points 1, 2 described in Table 1 to obtain twosubstrates. After the slicing, grinding and polishing were performed asin Case 1 of the present invention. Thereby, a GaN substrate of Case 3of the present invention was manufactured.

When distribution of an off angle was evaluated by performing X-raydiffraction measurement on the manufactured GaN substrate, it wasconfirmed that the GaN substrate had a front surface which wassubstantially the a-plane and did not include an off angle of zero.

Comparative Example 1

An AlN single crystal in a warped shape as in Case 1 of the presentinvention was prepared, and sliced with a wire saw along a sliced flatplane described in Table 1, which was different from the slicing methodsin Cases 1 to 3 of the present invention. After the slicing, grindingand polishing were performed as in Case 1 of the present invention.

When distribution of an off angle was evaluated by performing X-raydiffraction measurement on a manufactured AlN substrate, a substratewith the m-plane was obtained, but it was an AlN substrate having an offangle of zero which is not preferable for epitaxial growth.

Although the embodiment and examples of the present invention have beendescribed above, it is originally intended to combine features of theembodiment and examples as appropriate. Further, it should be understoodthat the embodiment and examples disclosed herein are illustrative andnon-restrictive in every respect. The scope of the present invention isdefined by the scope of the claims, rather than the embodiment describedabove, and is intended to include any modifications within the scope andmeaning equivalent to the scope of the claims.

DESCRIPTION OF THE REFERENCE SIGNS

10: nitride substrate, 11, 22 a: front surface, 12: first region, 13:second region, 13 a: first point, 13 b: second point, 17: raw material,21: base substrate, 22: nitride crystal, 22 b: rear surface, 100: growthapparatus, 101: crucible, 101 a: air exhaust outlet, 121: heat body, 121a, 123 a: inlet, 121 b, 123 b: outlet, 123: reaction vessel, 125:heating portion, 127 a, 127 b: radiation thermometer, T1, T2, T3, U1,U2, U3, W1, W2, W3, V1, V2, V3, V4: flat plane, O, O1, O2: center.

1. A method of manufacturing a nitride substrate, comprising the steps of: growing a nitride crystal; and cutting the nitride substrate including a front surface from said nitride crystal, wherein, in said step of cutting, said nitride substrate is cut such that an off angle formed between an axis orthogonal to said front surface and an m-axis or an a-axis is greater than zero.
 2. The method of manufacturing a nitride substrate according to claim 1, wherein, in said step of cutting, said nitride substrate is cut such that said nitride substrate includes said front surface having a first region and a second region surrounding said first region, and said off angle has a minimum value at a first point in said second region.
 3. The method of manufacturing a nitride substrate according to claim 2, wherein, in said step of cutting, said nitride substrate is cut such that said second region is within 2 mm from an edge of said nitride substrate.
 4. The method of manufacturing a nitride substrate according to claim 2, wherein, in said step of cutting, said nitride substrate is cut such that said off angle has a maximum value at a second point in said second region, and said off angle monotonically decreases from said second point to said first point.
 5. The method of manufacturing a nitride substrate according to claim 1, wherein, in said step of cutting, said nitride substrate is cut from said nitride crystal along a flat plane parallel to a flat plane inclined from an a-plane or an m-plane in a c-axis direction.
 6. The method of manufacturing a nitride substrate according to claim 1, further comprising the step of performing at least one of polishing and grinding of said front surface of said nitride substrate after said step of cutting.
 7. The method of manufacturing a nitride substrate according to claim 1, wherein, in said step of cutting, a plurality of said nitride substrates are cut.
 8. A method of manufacturing a nitride substrate, comprising the steps of: growing, in a c-axis direction, a nitride crystal including a front surface and a rear surface opposite to said front surface; and cutting the nitride substrate from said nitride crystal, wherein, in said step of cutting, said nitride substrate is cut from said nitride crystal along a flat plane which passes through said front surface and said rear surface of said nitride crystal and does not pass through a line segment connecting a center of a radius of curvature of said front surface with a center of a radius of curvature of said rear surface of said nitride crystal.
 9. The method of manufacturing a nitride substrate according to claim 8, wherein, in said step of cutting, said nitride substrate is cut such that said nitride substrate includes a front surface having a first region and a second region surrounding said first region, and an off angle formed between an axis orthogonal to said front surface and an m-axis or an a-axis has a minimum value at a first point in said second region.
 10. The method of manufacturing a nitride substrate according to claim 9, wherein, in said step of cutting, said nitride substrate is cut such that said second region is within 2 mm from an edge of said nitride substrate.
 11. The method of manufacturing a nitride substrate according to claim 9, wherein, in said step of cutting, said nitride substrate is cut such that said off angle has a maximum value at a second point in said second region, and said off angle monotonically decreases from said second point to said first point.
 12. The method of manufacturing a nitride substrate according to claim 8, wherein, in said step of cutting, said nitride substrate is cut from said nitride crystal along said flat plane parallel to a flat plane inclined from an a-plane or an m-plane in the c-axis direction.
 13. The method of manufacturing a nitride substrate according to claim 8, further comprising the step of performing at least one of polishing and grinding of a front surface of said nitride substrate after said step of cutting.
 14. The method of manufacturing a nitride substrate according to claim 8, wherein, in said step of cutting, a plurality of said nitride substrates are cut. 15-19. (canceled) 